This invention relates to devices for measuring rotational velocity and torsional vibration. More particularly, the present invention relates to such devices which employ Doppler shifts in reflected laser light.
The velocity and change in velocity of a moving surface constitute important process or operating parameters whose precise determination is often important or critical to the operation of equipment which is designed to be operated at specific speeds. While the instant invention is generally directed to the determination of rotational velocities, it will be readily understood that the principles discussed herein are equally applicable to measurement of other relatively flat moving surfaces including belts, conveyor lines, sheet material, and other similar bodies. In many such situations, it is important that the measurement device not be in actual physical contact with the body in motion. This may be due to the fact that the surface is moving at an extremely high speed or because of an otherwise harsh work environment. Additionally, non-contacting rotational velocity measurement devices are important in those circumstances in which it is necessary not to damage the workpiece as might occur if the moving surface were to be in contact with the measurement transducer.
A particularly useful area in which this invention functions is in the determination of the rotational velocity and torsional vibration associated with the rotors or large electrodynamic machines such as electrical generators employed to produce electrical power. These machines employ relatively strong magnetic fields. Thus, an electrical fault which occurs in the transmission system to which these generators are coupled, produces extremely high torques which tend to produce torsional oscillatory motion in the generator rotor. More particulary, the torsional vibration history of these rotors is useful in predicting their expected life and in otherwise analyzing the complex relationship that exists between the electrical generation facility and the power transmission portion of the utility system.
In the above-described situation, with respect to electrical generator rotors, it is seen that the rotational velocity and changes in this velocity, produced by torsional vibration, are the principal parameters of interest. However, in this situation and in others, it is also important to provide a means for measuring axial or thrust velocity. A unique feature of the instant invention permits the simultaneous determination of rotational (tangential) and axial velocities.
Other experiments in the field have also employed laser Doppler shift and in particular laser speckle (see below) Doppler shift to determine a single velocity from a moving surface. However, none of these experimenters employ surface properties to improve signal to noise ratio, to produce Bragg diffraction reflections or to measure motion in a plurality of directions. In general, such experiments appear to be limited to the analysis of "speckle". Speckle is the pattern of reflected illumination produced when a reflecting object is illuminated by light from a continuous coherent light source such as a laser. Speckle is an unordered and seemingly random pattern produced by microscopic features of the nominally smooth surface being illuminated. Nonetheless, it appears that the speckled pattern may be employed, particularly when the surface is moving, to determine velocity. If the surface moves with velocity v, the speckle pattern can be made to move with the same velocity and in the same direction by suitable positioning of the optical components. A laser speckle system is described, for example, in U.S. Pat. No. 3,604,804 issued to Carl M. Penney and Henry Hurwitz, Jr. on Sept. 14, 1971. The nature of speckle is also discussed in a paper by J. W. Goodman and published in the Journal of the Optical Society of America in Volume 66, No. 11, Nov. 1976 in a paper entitled, "Some Fundamental Properties of Speckle". Similarly, D. G. Simpson and D. G. S. Lamb in a paper dated 1977 entitled, "A Laser Doppler System for the Measurement of Torsional Vibration" published by the National Engineering Laboratory of the United Kingdom in NEL Report No. 639 employ a dual beam laser interferometer for the measurement of torsional vibrations of a rotating shaft. However, there is no diffraction grating present on the surface in the Simpson and Lamb apparatus nor is there any attempt therein presented for the simultaneous measurement of tangential and axial motions; this is also true of the patent to Penney, et al. and the Goodman paper.